Flaws in the scheme include subsidence, which means 50 per cent of the shallow foundations will need to be rebuilt, cracks in the concrete beams and water ingress. This has lead to bumpy and uneven tracks which the authors say raises "catastrophic" safety concerns.

Cracks have appeared in the busway scheme - now Smarter Cambridge Transport are suggesting the whole scheme should be scrapped

The true cost

Once the £36.5 million repairs are carried out the true cost of the busway will reach more than £200 million, way above the £73 million the scheme was originally expected to cost.

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An independent report by Smarter Cambridge Transport into the busway's defects reads: "Local taxpayers will have to foot any future bill and suffer the disruption and loss of amenity while further repairs are carried out. Therefore, any remedial action must resolve all design issues now.

"This will entail closing sections of the busway for extended periods, during which buses will have to be routed onto other roads. The lowest cost estimate, based on work being carried out proactively, is £36.5m. The cost rises to over £160m if repairs are carried out reactively.

"There may be a high risk of future structural failure of the busway. In particular, the risk of a catastrophic failure of the busway, which could lead to deaths or serious injuries... possibly within five years. This would create additional, currently unquantified, liabilities for the County Council and further prolonged periods of closure."

The authors added that they will recommend the council consider "cost-benefit analyses of alternatives to repairing the busway".

Suggestions include converting the busway to a restricted-access road or a light rail line.

It is six years from the pfficial opening of guided busway (pictured 2011) - where did it all go wrong?

Plagued with problems

There have been two derailments of single-decker buses on the Cambridgeshire Guided Busway since it launched in 2011, which have been attributed to driver error, but the report also threw up some problems with the kerbed track system.

This includes vehicles having to slow down at light-controlled intersections in order to re-engage safely with the guideway.

It is believed Cambridgeshire County Council has been aware of the serious defects since October 2014.

A spokeswoman for the council said: “The county council has agreed to proceed with legal action against BAM Nuttall in respect of defects in the design and construction of the guided busway.

"Our legal and technical advisers are preparing the county council’s formal claim, which will be a public document, so it would be inappropriate to comment in advance of that.

“However we can confirm that we have received a copy of the report produced by the Smarter Cambridge Transport team and do not consider that it raises any new issues that need to be considered as part of our claim.”

The News also contacted the busway's main contractor, BAM Nuttall, but the firm said it could not comment at present.